ECSE-6660Introduction to Optical Networking & Relevant Optics Fundamentals http://www.pde.rpi.edu/ Or http://www.ecse.rpi.edu/Homepages/shivkuma/ Shivkumar Kalyanaraman Rensselaer Polytechnic Institute email@example.com Based in part on textbooks of S.V.Kartalopoulos (DWDM) and H. Dutton (Understanding Optical communications), and slides of Partha Dutta
Quick History • Relevant Properties of Light • Components of Fiber Optic Transmission and Switching Systems • Chapter 2 of Ramaswami/Sivarajan Overview
Quick History of Optical Networking • 1958:Laser discovered • Mid-60s: Guided wave optics demonstrated • 1970: Production of low-loss fibers • Made long-distance optical transmission possible! • 1970: invention of semiconductor laser diode • Made optical transceivers highly refined! • 70s-80s: Use of fiber in telephony: SONET • Mid-80s:LANs/MANs: broadcast-and-select architectures • 1988: First trans-atlantic optical fiber laid • Late-80s:EDFA (opticalamplifier) developed • Greatly alleviated distance limitations! • Mid/late-90s:DWDM systems explode • Late-90s: Intelligent Optical networks
What is Light? Theories of Light Historical Development
What is Light? • Wave nature: • Reflection, refraction, diffraction, interference, polarization, fading, loss … • Transverse EM (TEM) wave: • Interacts with any charges in nearby space… • Characterized by frequency, wavelength, phase and propagation speed • Simplified Maxwell’s equations-analysis for monochromatic, planar waves • Photometric terms: luminous flux, candle intensity, illuminance, Luminance… • Particle nature: • Number of photons, min energy: E = hu • “Free” space => no matter OR EM fields • Trajectory affected by strong EM fields
Light Attributes of Interest • Dual Nature: EM wave and particle • Many s: wide & continuous spectrum • Polarization: circular, elliptic, linear: affected by fields and matter • Optical Power: wide range; affected by matter • Propagation: • Straight path in free space • In matter it is affected variously (absorbed, scattered, through); • In waveguides, it follows bends • Propagation speed: diff s travel at diff speeds in matter • Phase: affected by variations in fields and matter
Goal: Light Transmission on Optical Fiber Need to understand basic ideas of interacts with s and with matter
Interaction with Matter: Ray Optics • Light rays travel in straight lines
Reflection Applications: Mirrors & MEMS Plane Paraboloidal Elliptical Spherical
Ray Deflection by Prism • Newton’s Rainbow: Deflection angle dependent on the wavelength; • Used in optical multiplexers and de-multiplexers !
Internal & External Reflections • Critical Angle for Total Internal Reflection:
Total Internal Reflection • Total internal reflection forms the back-bone for fiber optical communication
Geometrical Optics: Fiber Structure • Fiber Made of Silica: SiO2 (primarily) • Refractive Index, n = cvacuum/cmaterial • ncore >ncladding • Numerical Aperture: Measures light-gathering capability n~1.43 n~1.45
Light Coupling into a fiber Effect of numerical aperture…
Geometrical Optics Applied to Fiber • Light propagates by total internal reflection • ModalDispersion: Different path lengths cause energy in narrow pulse to spread out • T = time difference between fastest and slowest ray
Total Internal Reflection & Modes • Impacts how much a fiber can be bent! • Micro-bends can eat up energy, kill some modes! • Modes are standing wave patterns in wave- or EM-optics!
EM Optics: Optical Electromagnetic Wave Linear polarization assumed …
Speed of Light in a Medium As a monochromatic wave propagates through media of different refractive indices, its frequency remains same, but its velocity, wavelength and wavenumber are altered.
Diffraction or Fresnel Phenomenon Cannot be explained by ray optics!
Diffraction Grating • Periodic thickness or refractive index variation (“grooves”) * Diffraction also occurs w/ pin hole of size of ~ * In polychromatic light, different wavelengths diffracted differently
Interference: Young’s Experiment Interference is simple superposition, and a wave-phenomenon
Multiple Waves Interference (Equal Amplitude, Equal Phase Differences) Sinc-squared function
High Intensity, Narrow Pulses from Interference between M Monochromatic Waves • Used in Phase locked lasers
Optical Splicing Issues: Speckle Patterns Speckle patterns are time-varying and arise from solution of Maxwell’s equations (> geometric optics)
Optical Transmission: More Light-Matter Interaction Effects Attenuation Dispersion Nonlinearity Reflectance Transmitted data waveform Waveform after 1000 km
Absorption vs Scattering Both are linear effects that lead to “attenuation”. Rayleigh scattering effects dominate much more than absorption (in lower Wavelengths, but decreases with wavelength)
Absorption and Attenuation: Absorption Spectrum Material absorption (Silica) 0.2 dB/km
Fiber:Transmission Windows Lucent’s new AllWave Fiber (1998) eliminates absorption peaks due to watervapor in the 1400nm area!